The invention relates to an aluminum alloy that is not sensitive to quenching, and which is used for the production of high-strength forged pieces low in inherent tension, and high-strength extruded and rolled products. Furthermore, the invention relates to a method for the production of a semi-finished product from such an aluminum alloy.
High-strength aluminum alloys are needed for the aeronautics and space industry, in particular, bearing hull, wing, and chassis parts which demonstrate high strength both under static stress and under dynamic stress. The required strength properties can be achieved, in the case of the aforementioned semi-finished products, by using alloys from the 7000 group (7xxx alloys), in accordance with the classification of aluminum alloys prepared by the Aluminum Association (AA).
Die-forged pieces for parts that are subject to great stress in the aeronautics and space industry, for example, parts made from the alloys AA 7075, AA 7175, AA 7475 and, particularly preferably, from the alloys AA 7049 and AA 7050, in America, and made from the alloys AA 7010, AA 7049A, and AA 7050A in Europe.
A high-strength aluminum alloy of the aforementioned type is known from WO 02/052053 A1, or U.S. Pat. No. 6,972,110 issued on Dec. 6, 2005 to Chakrabarti et al., the disclosure of which is hereby incorporated herein by reference. That reference discloses an alloy having an increased zinc content as compared with earlier alloys of the same type, coupled with a reduced copper and magnesium content. The copper and magnesium content in the case of this previously known alloy amounts to less than 3.5%, in total. The copper content itself is indicated as being 1.2-2.2 wt.-%, preferably 1.6-2.2 wt.-%. In addition to the elements zinc, magnesium, and copper, this previously known alloy necessarily contains one or more elements from the group zirconium, scandium, and hafnium, with maximum proportions of 0.4 wt.-% zirconium, 0.4 wt.-% scandium, and 0.3 wt.-% hafnium.
The semi-finished products should be subjected to a special heat treatment to produce the semi-finished products from one of the aforementioned alloys. These products can be in the form of forged pieces, wherein with this heat treatment, the extruded profiles, or the rolled sheets are treated to have the desired strength. This treatment includes quenching from solution heat temperature, in most cases combined with subsequent cold forming at medium thickness values of more than 50 mm. The cold forming serves to reduce the tensions induced during quenching. The step of cold forming can occur by means of cold upsetting or also by means of stretching the semi-finished product, typically by 1-3%. The semi-finished products produced should be as low in inherent tension as possible, to minimize any undesirable drawing during further processing. In addition, the semi-finished products and also the finished parts produced from them should be low in inherent tension, to give the designer the possibility of utilizing the entire material potential. For this reason, the method steps to be used for the production of parts for aeronautics and space technology from the alloys AA 7050 as well as AA 7010, and also the maximum thickness of the semi-finished products used for the production of the parts, are standardized and/or prescribed. The maximal permissible thickness is 200 mm and presupposes that after quenching, the semi-finished product is necessarily subjected to a cold forming step, for the reasons indicated above. With extruded and rolled products, cold forming can be achieved in a fairly simple manner, because of the geometry, which is generally simple, via stretching in the longitudinal direction. With geometrically complicated forged pieces, on the other hand, it is only possible to achieve a uniformly high degree of upsetting with great effort and expense, if it is even possible at all. In the course of designing larger aircraft, larger and larger and, in particular, thicker and thicker forged parts are constantly required.